APS FEL Achieves Ultraviolet Saturation

The Advanced Photon Source (APS) low-energy
undulator test line (LEUTL) has achieved "saturation" of
self-amplified spontaneous emission in a mirrorless free-electron
laser at a wavelength over 1000 times shorter than the previous
record. This important accomplishment demonstrated that such
free-electron lasers based on this process may one day provide
laser-quality x-ray beams and possibly open exciting new horizons for
research in dozens of scientific fields.

Above: Measured intensity of the 530 nm optical signal
as a function of distance
down the undulator. The
points are the measured data and the solid line is the
simulation result.

The beam of light produced in the experiment
had a wavelength of 385 nanometers, placing it in the ultraviolet
region of the spectrum. The success of the particular process
employed the APS is gauged by whether the free-electron laser effect
has "saturated," meaning the point at which the maximum power has
been yielded by the electron beam and converted to coherent
synchrotron radiation. The APS device clearly exhibited saturation of
the process.

The next generation of x-ray sources for
scientific research will be based on the free-electron laser concept,
the latest extension and refinement of synchrotron radiation. Unlike
a more conventional laser the APS free-electron laser uses a powerful
electron accelerator in combination with long arrays of very precise
magnets of alternating polarity and needs no mirrors for operation. A
further significant feature of this free-electron laser is that by
merely changing the electron beam energy it is continuously tunable
over a broad range of wavelengths thus breaking additional barriers
to traditional lasers.

Today, x-rays are the most widely used
scientific probe for studying the structures and interactions of
crystalline materials at the atomic and molecular levels. But many
materials do not form crystals, and many reactions take place too
quickly to study adequately, even at the APS. With further
development, free-electron lasers of this sort promise to provide
extremely bright, laser-like x-ray beams with ultrashort pulse
durations that will enable scientists to study the properties and
structures of materials in far greater detail and in far less time
than is possible today.

As presently configured the Advanced Photon
Source free-electron laser should be capable of reaching wavelengths
as low as 60 nanometers with the primary limitation being the energy
of the linear accelerator. To reach x-ray wavelengths one needs
electron beam energies comparable to what can be produced by the
world's largest linear accelerator located at the Stanford Linear
Accelerator Center (SLAC). However, to capitalize on this national
resource a host of additional technical challenges must still be
solved.

Argonne is one of six U.S. research
organizations collaborating on developing the free-electron laser
technology needed to achieve this national goal of an x-ray
free-electron laser. Others in the collaboration include the Stanford
Linear Accelerator Center (SLAC), the University of California at Los
Angeles, and Brookhaven, Los Alamos, and Lawrence Livermore national
laboratories. Their collaboration is aimed at demonstrating the
feasibility of the proposed Linac Coherent Light Source, a
proof-of-principle fourth-generation X-ray light source to be built
at SLAC.